[lachlangrose]

Summary

• Linter fix. ubuntu-latest is now Ubuntu 26.04. actions/setup-python has no Python 3.9 build for it, so every linter run failed at Set up Python (The version '3.9' with architecture 'x64' was not found for Ubuntu 26.04). This also caused the failures on chore(deps): bump nbconvert from 7.17.1 to 7.17.2 #353 and test: cover from_processor with a stratigraphic column #297. The linter now uses Python 3.12. black and ruff still check against py39, because target-version in pyproject.toml sets it.
• Fewer duplicate runs. After a push of a stack of branches, 60+ runs were in the queue. Many were old copies for the same branch.
  • linter, packages, tester and qgis-compat now have a concurrency group for each workflow and ref. A new PR run cancels the older one. Every push to master still runs to the end.
  • The linter ran on push to every branch and also on pull_request, so each PR branch got linted twice. The push trigger now applies to master only.
• The linter now also runs when linter.yml changes.

Not changed:

• qgis-compat.yml also asks for 3.9, but it uses setup-uv, which downloads its own Python builds.
• CodeQL uses GitHub's default setup, so you configure it in Settings → Code security, not in this repo.

Testing

• The linter passed on the first commit of this PR. All steps succeeded.
• The workflow YAML parses.

🤖 Generated with Claude Code